The invention relates to a braking device for a high-inertia rotor.
A high-power water-driven alternator is usually mechanically braked by friction between a steel track integral with the rotor against brake linings moved by the pistons of stationary cylinders.
The friction forces which appear when the brakes are applied are designed to be sufficient to overcome both the angular momentum of the rotor and the residual motor torque of the turbine due to leakage from its inlet manifold even when in the closed position, in order to slow the rotor down in a time which is compatible with the operation of the machine. The quantity of heat produced by friction is proportional to the product of the friction forces multiplied by the distance traveled by a point of the track of the rotor relative to a stationary observer.
The dissipation of this quantity of heat sets problems which are tricky to solve mainly on very powerful machines in which the rotor inertia is considerable and in which the water inlet manifold which feeds the turbine allows a high leakage flow to pass even when it is required to close the manifold completely to stop the alternator. The difficulty resides in the fact that heat is produced very much more rapidly than it can be dissipated naturally, in which case if suitable precautions are not taken, there occur local heating, deformation, excessive wear, and pollution of the machine by powdered material worn off the track and off the linings of the brake cylinders.
This is due to the fact that the coefficient of heat exchange between the metal of the track and the air is low. Also the heat emitted on the friction surface is stored in the mass of the track and flows only slowly in the ambient air.
In most known devices the tracks are constituted by solid steel segments and heat accumulates in their mass, but since the heat conductivity of steel is poor, the distribution of the temperatures is not homogenous. This limits the uses to which such tracks can be put.
Preferred embodiments of the present invention provide a braking device for a high-inertia rotor by which an increased braking capacity can be obtained without danger of damage.